Micro-electro-mechanical systems (MEMS) arc increasingly used in optical switching and scanning applications. Visual displays based on switchable MEMS micromirrors capable to withstand billions of switching cycles are now common. Using MEMS devices in fiberoptic switches attracts a particular interest. Light emitted by optical fibers can be tightly focused, which facilitates utilization of MEMS micromirrors to reliably switch optical signals between different optical fibers or waveguides.
In recent years, multiport wavelength-selective optical switches have been used to provide wavelength-specific switching of optical signals between different optical ports. To increase the number of optical ports in a wavelength-selective optical switch, there has been a tendency to focus optical beams reflected by MEMS micromirrors tighter and tighter, down to a value limited by diffraction. It is a well known principle of optics that a tighter focusing requires a larger beam size before focusing, to reduce the diffraction limit value. This calls for larger MEMS micromirrors to be able to redirect larger optical beams. To ensure good quality of a reflected optical beam, the mirrors have to be very flat. To keep the mirror flatness at a larger mirror size, the MEMS micromirrors have also to be made thicker.
Increased thickness of MEMS micromirrors, however, causes another problem to occur. The problem is related to dynamic performance of MEMS micromirrors. It takes longer to tilt larger, bulkier MEMS micromirrors because of increased moment of inertia (also called mass moment of inertia or rotational inertia) of the MEMS micromirrors. Furthermore, increased mass and moment of inertia increases sensitivity of MEMS mirrors to shock and vibration. These highly detrimental effects could be overcome by increasing stiffness of torsional hinges used to suspend MEMD micromirrors. However, increasing the stiffness of hinges requires increasing electrostatic torque created by MEMS actuators to offset the increased spring force of stiffer hinges. Unfortunately, there is a limit to a magnitude of the torque that can be generated, due to geometrical and electronic driver limitations.
A general approach used in the prior art to solving the problem of reducing mirror mass is to make the mirrors hollow and/or to provide “rigidity ribs” or truss structures to reinforce the larger MEMS mirrors. By way of example, Dewa in U.S. Pat. No. 6,704,132 incorporated herein by reference, discloses a micromirror having a plurality of truss members disposed under a gimbal portion of the micromirror, allowing the gimbal and mirror portions to be made of a thinner material, thereby reducing the mass and increasing the resonant frequency of the micromirror device. Sniegowski et al. in U.S. Pat. No. 6,791,730, incorporated herein by reference, discloses a reinforced mirror microstructure, in which adjacent structural layers are interconnected by a plurality of vertically disposed columns, or a plurality of laterally extending rails or ribs.
Moidu in U.S. Patent Application Publication 20080018975, incorporated herein by reference, discloses a large “micromirror”, for example 3 mm by 4 mm, having sufficient rigidity to ensure a low mirror curvature, for example a radius of curvature greater than 5 m, and a high resonance frequency of greater than 1 kHz. The micromirror of Moidu has a honeycomb structure sandwiched between two solid and smooth silicon layers.
One drawback of the ribs and honeycomb-reinforced MEMS micromirror structures of the prior art is the difficulty of manufacturing complex three-dimensional structures. For example, the honeycomb structure of Moidu, although providing a very good stiffness to rotational inertia ratio, requires multiple wafer stacked together to form the honeycomb core and skins, thus increasing manufacturing complexity and cost.
The prior art is lacking a large, for example more than 1 mm in size, MEMS mirror having a high quality of its reflective surface and low moment of inertia, that would also be relatively easy to manufacture. Accordingly, it is a goal of the invention to provide such a MEMS mirror, as well as a method of lessening the moment of inertia of a MEMS mirror, while preserving a high quality of its reflective surface, for example low curvature of the surface. A high quality of a MEMS reflective surface results in a high quality of an optical beam reflected from that surface, and ultimately in an improved performance of an optical device the MEMS mirror is used in.